Hydrocarbons, such as oil and gas, are commonly obtained from subterranean formations that may be located onshore or offshore. The development of subterranean operations and the processes involved in removing hydrocarbons from a subterranean formation typically include a number of different steps such as, for example, drilling a wellbore at a desired well site, treating the wellbore to optimize production of hydrocarbons, and performing the necessary steps to produce and process the hydrocarbons from the subterranean formation.
Certain subterranean reservoirs contain hydrocarbons, which are difficult to produce because they are highly viscous. Tar sand formations are one example of such a reservoir. One common technique for recovering oil and gas from such subterranean reservoirs is to inject them with steam. The steam makes the hydrocarbons less viscous thereby making them easier to produce through conventional production casing or tubing. There are several different methods for injecting steam into the formation. One such manner is simple injection of steam into a wellbore and producing from a nearby or adjacent wellbore. The other is by use of a Huff & Puff well. A Huff & Puff well has the advantage of simply requiring a single well and thereby avoids the cost and expense of drilling multiple wells.
A drawback, however, of Huff & Puff wells, and other steam injection wells is that the wide temperature and pressure variations that are generated through the steam injection process and subsequent cooling of the well to allow production to flow puts stress on the cement bonds that are formed between the casing string and the wellbore. This is because the casing string itself expands and contracts in response to the temperature and pressure variations. Over time, this expansion and contraction of the casing string can result in a failure of the bond formed between the casing string and the wellbore, which can be detrimental to the structural integrity of the well and to the hydraulic seal formed by the cement.
Studies have found that if the casing string is pre-stressed, for example, by being put under tension prior to cementing to the wellbore, it can better withstand the wide temperature and pressure swings that occur with the steam injection process. This is because the pre-stressing of the casing string limits the expansion and contraction that occurs with the temperature and pressure swings.
One technique that has been developed to pre-tension the casing string involves employing two different types of cement slurries, each having different set times. The first step in this process is to pump the slurry having the longer set time, known as the lead slurry, down the casing string after it has been installed in the wellbore and back up the annulus formed between the casing string and the wellbore. The next step is to pump the slurry with the shorter set time, known as the tail slurry, behind the lead slurry. The tail slurry is pumped down the casing string and back up the annulus. It is placed along the bottom portion of the annulus, for example, along the bottom 500 feet in a 2,000-foot well. Once the tail slurry sets, rigidly securing the bottom portion of casing string to the formation, then the rig pulls up on the top of the casing string, the casing string is thereby put into tension. Slips are then set at the surface to hold the casing string in tension as the lead slurry sets. Once both slurries have set, the casing string remains bonded in place under tension.
While this technique puts the casing string in a pre-stressed condition and thereby minimizes the cement bond failures that would otherwise occur without pre-loading, it has the drawback of requiring the rig to remain idle while the tail slurry sets. This results in lost rig time of approximately 5 hours or more for each cement job performed. In fields having hundreds or thousands of wells, this can be quite costly for the well operator.
The present disclosure is directed to a method and apparatus that seeks to pre-stress the casing string while minimizing the costly rig time required with current pre-tensioning techniques.